I. Field of the Invention
The present invention relates to a method and related apparatus for reading out a plurality of amplified signals, such as the output signals from a photodiode array, in either a high or lower resolution mode.
II. Background Information
There are many instances in which a plurality of signals need to be amplified and read out sequentially. For example, output signals from an array of photodiodes can be individually amplified and thereafter made available to acquire a composite signal indicative of the operation of the individual photodiodes. Such composite signals may, for example, be used to acquire an image corresponding to light incident upon the photodiodes.
FIG. 1 is a block diagram illustrating conventional circuitry for sequentially reading signals generated by a photodiode array having four diodes. The circuit of FIG. 1 includes a photodiode array 102 comprising a plurality of photodiodes 104, 106, 108 and 110, a plurality of amplifiers 112, 114, 116, 118, a plurality of sample and hold circuits 120, 122, 124 and 126, an analog multiplexer 128, and an output bus 130. The output of each photodiode 104, 106, 108 and 110 is connected to the input of a corresponding amplifier 112, 114, 116 and 118. The output of each amplifier 112, 114, 116 and 118 is coupled to a respective input of sample and hold circuits 120, 122, 124 and 126. The output of each sample and hold circuit 120, 122, 124 and 126 is coupled to an input of analog multiplexer 128. Multiplexer 128 has as a control input select lines 140. The output of analog multiplexer 128 is coupled to output bus 130.
The combination of photodiode 104, amplifier 112 and sample and hold circuit 120 forms a first channel 132. The combination of photodiode 106, amplifier 114 and sample and hold circuit 122 forms a second channel 134. The combination of photodiode 108, amplifier 116 and sample and hold circuit 124 forms a third channel 136. The combination of photodiode 110, amplifier 118 and sample and hold circuit 126 forms a fourth channel 138.
In operation, each of photodiodes 104, 106, 108 and 110, as is known to those skilled in the art, generates an output current as a function of light incident thereon. This output current is integrated by the corresponding amplifiers 112, 114, 116 and 118 to supply a corresponding voltage which is held in respective sample hold circuits 120, 122, 124 and 126 until delivered to bus 130 through operation of analog multiplexer 128. Specifically, analog multiplexer 128 is controlled through select signals 140 that operate, as is well known to those skilled in the art, to selectively couple an output from one of sample and hold circuits 120, 122, 124 and 126 to output bus 130. For example, during a complete read out cycle, select lines 140 may be cycled through states (00), (01), (10), and (11). Each of the states is maintained for a given time period. These states result in the corresponding connection of sample and hold circuit 120, 122, 124 and 126 to output terminal 130. Thus, signals representing light incident on photodiodes 104, 106, 108 and 110 are sequentially placed on output bus 130 through respective channels 132, 134, 136 and 138. The signal-to-noise ratio for each channel is dependent upon the characteristics of the individual operation of amplifiers 112, 114, 116 and 118.
Given the above method, four time periods are required to read an entire cycle of output signals from photodiode array 102. One conventional technique for improving readout speed is to sacrifice image resolution by simply skipping the outputs from one or more of channels 132, 134, 136 and 138. For example, by limiting select lines 140 to the states (00) and (10) for a given readout period, the readout speed may be doubled. However, with this method, the signal-to-noise ratio for each selected channel is the same as the signal-to-noise ratio for each channel with all channels accessed, and the information contained in the skipped channels is lost.